1. Field of the Invention
The present invention relates to a wireless local loop (WLL) system using a so-called "Personal Handy-phone System" (PHS), more particularly relates to a base transceiver station and a subscriber unit in a wireless local loop system and a method for operating the same.
Wireless local loop systems are currently being closely looked at among the various access type systems. They may be considered one of the optimal means for connecting a subscriber exchange (local exchange) with subscribers through radio channels.
One way for constructing such a wireless local loop system is to build upon the existing cellular networks used for cellular telephones.
Another way for constructing such a wireless local loop system is to build upon a new PHS network.
There are advantages and disadvantages to each of the above first and second techniques. The first technique is better from the viewpoint of the mobility of the subscriber (cellular telephone), that is, enables the cellular telephone to be used by a moving car, train, etc. On the other hand, the second technique is better from the viewpoint of the affinity and compatibility with existing public switched telephone networks (PSTN), that is, the cellular networks upon which the first technique is based exist as completely different systems from the public switched telephone networks, Further, the second technique is better from the viewpoint of the extremely high quality of the lines.
The present invention relates to a wireless local loop system established based on the second technique, that is, a wireless local loop system incorporating a PHS network.
2. Description of the Related Art
The basic technology for realizing the PHS type wireless local loop system upon which the present invention is founded already exists, as will be illustrated later, however, use of such a wireless local loop system by a large number of subscribers as part of a public switched telephone network in the broad sense of the term requires more than simple technical feasibility. It is further required that the wireless local loop system be practical.
Realization of such a practical wireless local loop system, however, requires that several tasks be solved. It is particularly important to solve the following six tasks:
1) Sharp reduction of the number of transmitting and receiving antennas which must be provided in the base transceiver station (BTS) so as to reduce the size and raise the efficiency of the base transceiver station. 2) Elimination of the problems inherent to conventional PHS networks, that is, the problem of the small service area per base transceiver system and the problem of the susceptibility to interference from other base transceiver stations.
3) Design of a configuration enabling full utilization of the inherent characteristic of a PHS network as mentioned above, that is, the excellent affinity and compatibility with a public switched telephone network.
4) Inexpensive realization of the functions of an integrated services digital network (ISDN) to eliminate the reliance of existing PHS networks on expensive ISDNs.
5) Minimalization of the deterioration in the quality of service to the subscriber due to the higher probability of the occurrence of faults in a base transceiver station accompanying the large number of cell stations (CS) required to be provided in the base transceiver station in the case of a cell with a large amount of traffic.
6) Relating to the task of 5), realization of a base transceiver station keeping the occurrence of the above faults to a minimum, having a high transmission quality, and having a high reliability.